Your niche is what will separate you from your competition.
To find your niche, begin by considering your interest. Chances are that if you have an interest in a subject, you have developed a considerable knowledge for that subject. Consider what you can do well.
If you are particularly good at decorating, cooking, playing golf, scuba diving, or finding antiques, then it may make sense to write about these subjects. If you are in a profession where you find people often turn to you for help, write on this topic. Look through books and magazines that you have purchased to see if there is a pattern of interest.
Keep in mind when making a list of your interest that it is important to select areas that would also be of interest to others. Just because it is an area of interest to you does not mean that it would draw a large market. You will not be able to sell your work if it is not going to be profitable for the publisher.